Premultiply FTW!

src/ImageSharp/Processing/Processors/Transforms/AffineProcessor.cs

@@ -199,12 +199,14 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                     for (int xx = 0, i = minX; i <= maxX; i++, xx++)
                                     {
                                         float xWeight = xSpan[xx];
-                                        sum += source[i, j].ToVector4() * xWeight * yWeight;
+                                        var vector = source[i, j].ToVector4();
+                                        var mupltiplied = new Vector4(new Vector3(vector.X, vector.Y, vector.Z) * vector.W, vector.W);
+                                        sum += mupltiplied * xWeight * yWeight;
                                     }
                                 }
 
                                 ref TPixel dest = ref destRow[x];
-                                dest.PackFromVector4(sum);
+                                dest.PackFromVector4(new Vector4(new Vector3(sum.X, sum.Y, sum.Z) / sum.W, sum.W));
                             }
                         });
             }

tests/ImageSharp.Tests/Processing/Transforms/TransformTests.cs

@@ -17,14 +17,14 @@ namespace SixLabors.ImageSharp.Tests.Processing.Transforms
         public static readonly TheoryData<float, float, float, float, float> TransformValues
             = new TheoryData<float, float, float, float, float>
                   {
-                      { 45, 1, 1, 20, 10 },
-                      { 45, 1, 1, -20, -10 },
-                      { 45, 1.5f, 1.5f, 0, 0 },
+                      { 50, 1, 1, 20, 10 },
+                      { 50, 1, 1, -20, -10 },
+                      { 50, 1.5f, 1.5f, 0, 0 },
                       { 0, 2f, 1f, 0, 0 },
                       { 0, 1f, 2f, 0, 0 },
                   };
 
-        public static readonly TheoryData<string> ResamplerNames = 
+        public static readonly TheoryData<string> ResamplerNames =
             new TheoryData<string>
                   {
                       nameof(KnownResamplers.Bicubic),
@@ -64,7 +64,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Transforms
             using (Image<TPixel> image = provider.GetImage())
             {
                 // TODO: Modify this matrix if we change our origin-concept 
-                var rotate = Matrix3x2.CreateRotation((float)Math.PI/4f);
+                var rotate = Matrix3x2.CreateRotation((float)Math.PI / 4f);
 
                 image.Mutate(c => c.Transform(rotate, resampler));
                 image.DebugSave(provider, resamplerName);
@@ -88,9 +88,9 @@ namespace SixLabors.ImageSharp.Tests.Processing.Transforms
                 var translate = Matrix3x2.CreateTranslation(tx, ty);
                 var scale = Matrix3x2.CreateScale(sx, sy);
                 Matrix3x2 m = rotate * scale * translate;
-                
+
                 this.Output.WriteLine(m.ToString());
-                
+
                 image.Mutate(i => i.Transform(m));
                 image.DebugSave(provider, $"R({angleDeg})_S({sx},{sy})_T({tx},{ty})");
             }
@@ -104,10 +104,12 @@ namespace SixLabors.ImageSharp.Tests.Processing.Transforms
             IResampler sampler = GetResampler(resamplerName);
             using (Image<TPixel> image = provider.GetImage())
             {
-                Matrix3x2 rotate = Matrix3x2Extensions.CreateRotationDegrees(45);
+                Matrix3x2 rotate = Matrix3x2Extensions.CreateRotationDegrees(50);
                 Matrix3x2 scale = Matrix3x2Extensions.CreateScale(new SizeF(.5F, .5F));
+                var translate = Matrix3x2.CreateTranslation(75, 0);
+
 
-                image.Mutate(i => i.Transform(rotate * scale, sampler));
+                image.Mutate(i => i.Transform(rotate * scale * translate, sampler));
                 image.DebugSave(provider, resamplerName);
             }
         }